This invention generally relates to casks for transporting nuclear materials to or from nuclear power plant facilities and is specifically concerned with a cask having circumferentially oriented fins for both dissipating heat generated by spent fuel rods contained within the cask and for providing a protective, watertight barrier around a layer of neutron-absorbing cement disposed within the wall of the cask.
Casks for shipping fuel rods to and from nuclear power plants are known in the prior art. Such casks generally include a transportable steel vessel that is cylindrical in shape, and a basket structure that is receivable within the steel vessel having an array of cells for holding rectangular storage containers. Each of these storage containers, in turn, may hold either a fuel rod assembly, or a spent fuel canister wherein fuel rods are consolidated in a dense, triangular-pitch arrangement. Such transportable casks may be secured onto the trailer of a tractor-trailer, and are typically used to ship spent fuel rods from a nuclear power plant to a permanent waste isolation site or a reprocessing facility in as safe a manner as possible. At the present time, relatively few of such shipping casks have been manufactured and used since most of the utility companies that own nuclear facilities have been able to store the spent fuel rods in the spent-fuel pools that were initially built into such reactor facilities. However, the availability of such on-site storage space is steadily diminishing as an increasing number of fuel assemblies are being loaded into the spent-fuel pools of such facilities everyday. The recognition of the need for additional storage facilities has induced the Congress of the United States to pass an act obligating the Nuclear Regulatory Commissioner (NRC) to move the spent fuel assemblies from the on-site storage facilities of nuclear power plants to a federally operated nuclear waste disposal facility starting in 1998. Thus the need for such casks is about to increase substantially.
While the transportation vessels of the prior art are generally capable of safely transporting spent fuel to a final destination, there is considerable room for improvement. But, before these potential areas of improvement can be fully appreciated, some understanding of the objectives that these casks must meet is necessary.
In order to be practical, a cask for transporting radioactive material by truck must meet at least five basic criteria. First, the walls of the cask must be capable of effectively shielding both the gamma and neutron radiation emitted by its payload so that the total amount of radiation emitted from the surface of the cask is at a level low enough to be safely handled. In fact, U.S. Nuclear Regulatory Commission (NRC) regulations specify that the surface radiation of any such cask may be no greater than 200 millirems at any given point, and that the radiation emitted by the cask be no greater than 10 millirems at a distance of two meters from any vehicle that the cask is mounted on. Secondly, the cask must be capable of withstanding the mechanical shock of a magnitude commensurate with that of a vehicular accident. In this regard, it is not enough that the walls of the cask continue to contain the radioactive material after such a mechanical shock. They must further maintain water tightness at all points so that water will not have an opportunity to leak into the interior of the cask and thermalize the neutrons being emitted by the spent fuel rods or other materials contained therein. Thirdly, the basket structure within the cask must be capable of withstanding the forces applied to its perimeter by the inner cask walls in the event of an accident without any significant distortion of its individual, waste-containing cells. If these cells do undergo significant amounts of distortion, the effectiveness of the neutron "traps" installed between these cells could be jeopardized, which could in turn result in a criticality condition within the cask. Fourthly, the cask must be immersible in water without the incursion of any outside water into the interior of the walls of the cask, and must further be completely drainable. The reason for this requirement is that such casks are often loaded and unloaded in the spend-fuel pools of nuclear facilities to reduce exposure of the operating personnel to potentially harmful radiation The water in such pools typically contains dissolved radionucleides, which, if allowed to seep into the crevices in the cask, or to deposit themselves into micropores on the cask surface, might prove difficult if not impossible to remove. The deposition of such radionucleides in the crevices and surface pores of the casks could well raise the surface radiation of the cask beyond DOT limits, thereby rendering the cask useless. Finally, the cask must be capable of effectively rejecting the heat of decay generated by the radioactive materials within it. If no effective heat rejection mechanism exists within the cask, the temperature within the cask could become high enough to generate dangerous levels of pressure.
Unfortunately, the simultaneous achievement of these five criteria is difficult, as the materials and mechanisms which implement one or the other criteria are often at cross purposes with one another. For example, the use of radially-projecting fins provides very good heat dissipation characteristics, but reduces the cask's ability to withstand large mechanical shocks and still maintain wall integrity. One of the best and most economical neutron shielding materials known is high-hydrogen cement. But, such cement is brittle, and generally incapable of maintaining integrity if exposed to the shattering forces of an accident condition. Lead, depleted uranium, and Boro-silicon.RTM. are also well known, effective gamma shielding materials. However, none of these materials has sufficient mechanical strength to withstand an accident condition alone. Moreover, none of these materials a is good heat conductor, and none of these materials is particularly easy to join to a structurally strong metal in a secure, integral fashion by welding or by any other known means in view of the large differences in their mechanical and metallurgical properties. While stainless steel has good structural and corrosion resistant properties, it is not a particularly good heat transfer medium, and is expensive. Additionally, the applicant has observed that the surface of stainless steel contains micropores that are capable of capturing both dissolved radionucleide and radioactive dust. Finally, while carbon steel is a good and inexpensive structural material having better head transfer properties than stainless steel, it is apt to corrode when exposed to water.
Clearly, what is needed is a transportation cask which reasonably fulfills each of the five necessary criteria in a design which is relatively easy and inexpensive to fabricate.